Strange urine-like smell...

I'm not sure if anyone can help me in this forum, but I'll describe the problem I'm having...

About 6 months ago, a disgusting, urine-like smell started coming from my heating vents...we rent a house, and live in the ground floor suite, and have neighbours upstairs. The smell is even worse in their suite. We had the landlord come over to check it out, he vacuumed out the vents (?) and found nothing. I don't know if this is a plumbing-related problem or not.... could a sewage pipe somewhere be leaking or something? When we first noticed the stink, we covered all the vents in our place with plastic, and haven't smelled anything since. But now that winter's here, we need to turn on the heat, and the stench will be everywhere...

My neighbour thinks the smell could be coming from urea formaldehyde? (We don't even know if this has been used in this house or not)

anyone know what this could be, or who we would need to call to investigate/fix this problem? Thanks in advance...

There could possibly be a dried up drain trap or an open clean-out fitting on the sewer line in the mechanical room, (where the HVAC unit is) and it is drawing in an odor and sending it into the ducts.
A professional should investigate and/or an air test should be done, it could be dangerous.

Thank you both for replying. No, my neighbour doesn't have any pets...at least, that they've told me about.

I do think we will have to get someone in to do something about it. I know this is going to be an issue for my landlord. The last time he came over to fix the problem by vacuuming the vents, he saw we had a pet gerbil, and said "you can't complain of a urine/rodent smell when you're keeping rodents". The thing is, our gerbil's cage is kept very clean, and we had only had the gerbil for about a week when he came over to investigate the smelly vent issue, which had been going on for over 2 months at the time...anyway, if it was the gerbil causing the smell, why wouldn't the smell be all over the house, instead of just coming out of the vents?

It's an old house, stuff breaking down all the time...and of course, not being fixed/replaced until after fighting with the landlord...and all the while rents still get increased. That's what you get for renting, I guess.